Hexose - WikiHQ

In chemistry, a hexose is a monosaccharide (simple sugar) with six carbon atoms.

Hexoses exist in two forms, open-chain or cyclic, that easily convert into each other in aqueous solutions. The most important example is glucose.

In linear form, an aldohexose has four chiral centres, which give 16 possible aldohexose stereoisomers (24), comprising 8 pairs of enantiomers. The linear forms of the eight -aldohexoses, in the Fischer projection, are

File:DAllose Fischer.svg | -Allose 000

File:DAltrose Fischer.svg | -Altrose 001

File:DGlucose Fischer.svg | -Glucose 010

File:Mannose.svg | -Mannose 011

File:DGulose Fischer.svg | -Gulose 100

File:DIdose Fischer.svg | -Idose 101

File:DGalactose Fischer.svg | -Galactose 110

File:DTalose Fischer.svg | -Talose 111

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Of these -isomers, all except -altrose occur in living organisms, but only three are common: -glucose, -galactose, and -mannose. The -isomers are generally absent in living organisms; however, -altrose has been isolated from strains of the bacterium Butyrivibrio fibrisolvens.

When drawn in this order, the Fischer projections of the -aldohexoses can be identified with the 3-digit binary numbers from 0 to 7, namely 000, 001, 010, 011, 100, 101, 110, 111. The three bits, from left to right, indicate the position of the hydroxyls on carbons 4, 3, and 2, respectively: to the right if the bit value is 0, and to the left if the value is 1.

The chemist Emil Fischer is said to have devised the following mnemonic device for remembering the order given above, which corresponds to the configurations about the chiral centers when ordered as 3-bit binary strings:

:All altruists gladly make gum in gallon tanks.

referring to allose, altrose, glucose, mannose, gulose, idose, galactose, talose.

The Fischer diagrams of the eight -aldohexoses are the mirror images of the corresponding -isomers; with all hydroxyls reversed, including the one on carbon 5.

Ketohexoses

A ketohexose is a ketone-containing hexose.

The important ketohexoses are the 2-ketohexoses, and the most important 2-ketose is fructose.

Besides the 2-ketoses, there are only the 3-Ketoses, and they do not exist in nature, although at least one 3-ketohexose has been synthesized, with great difficulty.

In the linear form, the 2-ketohexoses have three chiral centers and therefore eight possible stereoisomers (23), comprising four pairs of enantiomers. The four -isomers are:

File:DPsicose Fischer.svg | -Psicose

File:D-Fructose.svg | -Fructose

File:DSorbose Fischer.svg | -Sorbose

File:DTagatose Fischer.svg | -Tagatose

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The corresponding forms have the hydroxyls on carbons 3, 4, and 5 reversed. Below are

depiction of the eight isomers in an alternative style:

File:Psicose.png | -Psicose

File:D-fructose CASCC.png | -Fructose

File:D-sorbose.png | -Sorbose

File:Tagatose.png | -Tagatose

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File:L-psicose.png | -Psicose

File:L-fructose.png | -Fructose

File:Sorbose.png | -Sorbose

File:L-tagatose.png | -Tagatose

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3-Ketohexoses

In theory, the ketohexoses include also the 3-ketohexoses, which have the carbonyl in position 3; namely . However, these compounds are not known to occur in nature, and are difficult to synthesize.

The unequivocal synthesis and isolation of a 3-ketohexose, xylo-3-hexulose, through a rather complex route, was first reported in 1961 by George U. Yuen and James M. Sugihara.

Cyclic forms

Like most monosaccharides with five or more carbons, each aldohexose or 2-ketohexose also exists in one or more cyclic (closed-chain) forms, derived from the open-chain form by an internal rearrangement between the carbonyl group and one of the hydroxyl groups.

The reaction turns the group into a hydroxyl, and the hydroxyl into an ether bridge () between the two carbon atoms, thus creating a ring with one oxygen atom and four or five carbons.

If the cycle has five carbon atoms (six atoms in total), the closed form is called a pyranose, after the cyclic ether tetrahydropyran, that has the same ring. If the cycle has four carbon atoms (five in total), the form is called furanose after the compound tetrahydrofuran. The conventional numbering of the carbons in the closed form is the same as in the open-chain form.

If the sugar is an aldohexose, with the carbonyl in position 1, the reaction may involve the hydroxyl on carbon 4 or carbon 5, creating a hemiacetal with five- or six-membered ring, respectively. If the sugar is a 2-ketohexose, it can only involve the hydroxyl in carbon 5, and will create a hemiketal with a five-membered ring.

The closure turns the carboxyl carbon into a chiral center, which may have either of two configurations, depending on the position of the new hydroxyl. Therefore, each hexose in linear form can produce two distinct closed forms, identified by prefixes "α" and "β".

It has been known since 1926 that hexoses in the crystalline solid state assume the cyclic form. The "α" and "β" forms, which are not enantiomers, will usually crystallize separately as distinct species. For example, -glucose forms an α crystal that has specific rotation of +112° and melting point of 146 °C, as well as a β crystal that has specific rotation of +19° and melting point of 150 °C.